add missing memberwise initializers to Mixins (#52)

Author: theMomax

Sources/SymbolKit/SymbolGraph/Symbol/FunctionSignature/FunctionSignature.swift

@@ -30,6 +30,11 @@ extension SymbolGraph.Symbol {
          */
         public var returns: [DeclarationFragments.Fragment]
 
+        public init(parameters: [SymbolGraph.Symbol.FunctionSignature.FunctionParameter], returns: [SymbolGraph.Symbol.DeclarationFragments.Fragment]) {
+            self.parameters = parameters
+            self.returns = returns
+        }
+        
         public init(from decoder: Decoder) throws {
             let container = try decoder.container(keyedBy: CodingKeys.self)
             parameters = try container.decodeIfPresent([FunctionParameter].self, forKey: .parameters) ?? []

Sources/SymbolKit/SymbolGraph/Symbol/Swift/Extension.swift

@@ -44,6 +44,12 @@ extension SymbolGraph.Symbol.Swift {
             case typeKind
             case constraints
         }
+        
+        public init(extendedModule: String, typeKind: SymbolGraph.Symbol.KindIdentifier? = nil, constraints: [SymbolGraph.Symbol.Swift.GenericConstraint]) {
+            self.extendedModule = extendedModule
+            self.typeKind = typeKind
+            self.constraints = constraints
+        }
 
         public init(from decoder: Decoder) throws {
             let container = try decoder.container(keyedBy: CodingKeys.self)

Sources/SymbolKit/SymbolGraph/Symbol/Swift/Generics.swift

@@ -47,6 +47,11 @@ extension SymbolGraph.Symbol.Swift {
          There is a *conformance constraint* involving `S`.
          */
         public var constraints: [GenericConstraint]
+        
+        public init(parameters: [SymbolGraph.Symbol.Swift.GenericParameter], constraints: [SymbolGraph.Symbol.Swift.GenericConstraint]) {
+            self.parameters = parameters
+            self.constraints = constraints
+        }
 
         public init(from decoder: Decoder) throws {
             let container = try decoder.container(keyedBy: CodingKeys.self)